1. Field of the Invention
The invention relates to apparatus which processes an input signal, that comprises a plurality of interleaved pulse trains, in order to unambiguously de-interleave the individual pulse trains that together constitute that signal and to uniquely identify the emitter which transmitted each pulse train.
2. Description of the Prior Art
Continuously operating radar systems have seen increasing use in recent years. So much so, that currently the electrical environment surrounding many geographic areas--particularly those having significant population concentrations and/or those having some strategic importance--generally contain pulsed signals emanating from perhaps as many as 50 to 100 or more separate sources (emitters). The pulsed signals transmitted from these emitters add, at the output of a wideband detector in a receiver, to form a composite signal which may, today, possess a pulse rate of 1 million pulses/second. As the use of radar systems increases, this pulse rate is expected to increase at least ten fold over the next few years.
Often, particular pulsed signals existing within an environment must be identified so that one radar emitter can be distinguished from another.
Individual pulsed signals which comprise any composite signal are usually identified by a pulsed emitter identification system which contains a pulse analysis system. The pulse analysis system de-interleaves the composite signal into its constituent pulse trains. Unfortunately, pulse analysis systems known to the art possess serious drawbacks.
For example, many prior art pulse analysis systems disadvantageously expend considerable time in de-interleaving the composite signal into its constituent pulse trains and are thus unsuitable for real-time use. Specifically, in these systems, all the pulses comprising the composite signal are first sampled for a pre-defined sampling period, generally ranging from 100 milliseconds to 1 second, in order to collect a known amount of input data. Concurrently therewith, time of arrival (TOA) data for each pulse is determined and stored in a memory for subsequent processing. Once all these operations have been completed, the TOA data for every sampled pulse is subsequently analyzed, by a computer, to discern any repetitive pattern existing in the data. Each pattern identifies a particular emitter. Unfortunately, few, if any, of these patterns are known ahead of time. Moreover, these patterns often contain undesireable components occurring at the harmonics of each constituent emitter. To eliminate any ambiguities caused by harmonics, all these harmonics must be detected and then removed from the discerned pattern prior to any further analysis. Consequently, pattern detection based on the TOA data requires rather sophisticated analysis algorithms which consume an excessive amount of processing time and, as such, can not generally be undertaken in real-time.
In particular, owing to the complexity of the analysis algorithms, these pulse analysis systems known to the art usually can not complete the processing of all the TOA data during a sampling period, and hence are unable to keep up with the input data. As a result, sampling can not be resumed until the processing has been completed, and, hence, usually results in a gap, occurring between successive sampling periods, during which no input data is sampled. Even if two or more identical analysis systems were to be used in an alternating fashion to avoid such a gap, a discontinuity would likely be created in the data between each sampled section. This gap or discontinuity is usually of no consequence whenever continuous emitters are being received. However, such a gap or discontinuity can result in the total loss of detection of an emitter which is received for only a short period of time, either because that emitter is a "short-on-time" emitter, as discussed below, and/or due to the scan geometry associated with that emitter and the receiving antenna. Moreover, as the pulse rate of the composite signal increases--as it is expected to do during the coming years, these prior art systems will not be able to "keep pace" with the increasing pulse rate and will thereby lose increasingly larger amounts of input data. Hence, the future ability of these prior art systems to both de-interleave the constituent pulse trains and to accurately identify emitters, particularly "exotic" emitters, will markedly decrease.
In order to reduce the processing needed to detect a pulse train existing within the composite signal and identify its associated emitter, the art teaches that various characteristics, i.e. the electrical "signature", of each previously identified emitter, and in particular the characteristics of its associated pulse train (e.g. its pulse repetition interval--PRI, whether that PRI varies and the magnitude of any such variation) can be stored in an "emitter file." With this information, data which describes each received pulse, collectively referred to as pulse descriptor data, is compared with each entry in the emitter file in an effort to quickly classify a pulse train and identify its associated emitter based upon its electical signature. This technique saves considerable time in identifying known emitters.
Unfortunately, some radar emitters intentionally change their characteristics over a wide range and often within a short period of time to avoid their detection by pulse analysis equipment. Such emitters are often referred to as "exotic" emitters. Illustrative types of such emitters, grouped in terms of increasing complexity, include: staggered emitters--i.e. those emitters which simultaneously and continuously generate a pulse train that has a pulse repetition interval that alternates between two values; multi-legged staggered emitters, i.e. those emitters which continuously generate a repetitive sequence of pulses each having a different PRI; wobulated emitters, i.e. those emitters which continuously produce pulses that have a PRI that varies in accordance with a continuous time dependant function, such as a sinusoid or a sawtooth; and various combinations of these types.
Furthermore, whenever a doppler radar emitter appears in any environment, its pulse train usually dominates the environment to the point of obscuring pulse trains produced by other emitters. This occurs because a doppler emitter continuously produces pulses having a very short fixed PRI. Since a doppler emitter produces a substantial number of pulses, these pulses are extremely difficult to filter out of the composite signal. For that reason, prior art pulse detection systems do not perform this filtering and hence experience difficulty in detecting non-doppler pulse trains which simultaneously appear with a doppler pulse train. This difficulty worsens considerably whenever two or more doppler emitters simultaneously transmit pulses into the same environment.
In addition, many exotic emitters utilize a "short-on-time". Short-on-time type radar emitters only produce a burst of pulses during a very short interval of time and then remain inactive (quiet) for a long interval of time. The number of pulses comprising any such burst is often small, generally ranging between 7 to 10. In fact, during any such burst, a short-on-time radar emitter will transmit just the minimum number of pulses necessary to obtain accurate distance information. Since short-on-time radars emit very few pulses per burst, these radars are extremely difficult to detect. Similar performance can also occur whenever a narrow beam search radar momentarily sweeps over the receiving antenna of the pulse analysis system.
Moreover, to further complicate pulse train detection and emitter identification, the composite signal often contains pulsed signals that have jittered pulse intervals, missing pulses and/or general background noise pulses.
Since the manner in which an exotic emitter changes and the amount of its change are generally unknown to any emitter identification system, comparing TOA and/or other pulse descriptor data against entries in an "emitter file" often provides erroneous results and thus can not be relied upon. Hence, few, if any, a priori assumptions can be made, based upon previously detected patterns, to accurately predict future performance of an exotic emitter. Moreover, a pulse analysis technique which detects pulse trains having a constant PRI can not be used to detect pulse trains emanating from exotic emitters. Consequently, to accurately identify the pulse trains from exotic emitters, all the TOA data has to be continuously analyzed using highly sophisticated pattern recognition algorithms to discern any repetitive patterns existant therein. No processing time can be saved by relying upon previously detected patterns. Thus, emitter identification systems known to the art require a substantial amount of processing time to detect a pulse train emanating from an exotic emitter. Furthermore, as the number, pulse rate and/or complexity of constituent pulse trains comprising any composite signal increases, the pattern recognition algorithms must be modified in order for them to handle these pulse trains. These modifications usually entail increasing the sophistication of these algorithms which, in turn, renders them much too slow to execute in real-time.
In addition, many radar emitters transmit pulses using a spread-spectrum technique, whereby the carrier frequency of a pulse stream varies, often widely, with time. With such a technique, one pulse in the stream is transmitted on one carrier frequency while the next pulse may be transmitted on a markedly different carrier frequency. Pulse analysis systems known to the art have generally utilized a channelized approach in analyzing spread spectrum pulse streams. Here, a wide spectral band is broken into separate frequency ranges. Each frequency range is analyzed by a different processor in order to identify its constituent pulse trains. Such an approach reduces the total number of pulses that need to be analyzed by each processor to those pulses which exist in the corresponding channel associated with that processor. Difficulties arise with channelized pulse analysis systems inasmuch as the separate pulses belonging to a spread spectrum pulse train often reside in different frequency ranges, and each frequency range is generally analyzed independently of any other frequency range. The results for one frequency range are not combined with the results for any other range to determine the existence of a spread-spectrum signal that extends over multiple frequency ranges. Thus, channelized pulse analysis systems are often unable to detect and identify spread-spectrum pulse trains.
Moreoever, pulse analysis systems known to the art have poor immunity to spurious noise pulses and hence possess a rather high noise floor. As such, these systems are often unable to detect fairly weak pulsed signals, particularly those which contain missing pulses.
For these reasons, known pulsed emitter identification systems are generally unsuitable for use in many applications.
Hence, a need exists in the art for apparatus which can unambiguously de-interleave a composite signal, comprising a substantial number of interleaved pulse trains produced by stable and/or exotic emitters of any type and complexity, into its constituent pulse trains in real-time and which can uniquely identify the emitter which transmitted each pulse train without the need to make any a priori assumptions about the characteristics of any such emitter.